Nonlinear radar exploits the difference in frequency between radar waves that illuminate electromagnetically nonlinear targets and the waves that reflect from those targets. Nonlinear radar differs from traditional linear radar by offering high clutter rejection when detecting a variety of man-made targets. Its disadvantage, compared with traditional linear radar, is that the power-on-target required to generate a comparable signal-to-noise ratio (SNR) is much higher. Nevertheless, nonlinear radar is particularly suited to the detection of devices containing metals and semiconductors that, while highly reflective, still possess a thin linear radar cross section.
Certain nonlinear targets may be tailored for maximum response in an intended frequency band Examples include tags for tracking insects, and tags worn by human beings for avoiding collisions with vehicles or for monitoring vital signs. Such tags contain a radio-frequency (RF) nonlinearity, often a Schottky diode, connected to an antenna that is sized for the intended frequencies of operation.
Other targets are not intended to respond to a nonlinear radar but do still respond because they contain nonlinearities inherent to their design, such as metal contacts, semiconductors, transmission lines, antennas, filters, connectors, and ferroelectrics RF electronic devices such as handheld radios and cellular telephones contain many nonlinear components and are therefore visible to a nonlinear radar, although the full range of frequencies at which a particular target responds is not known until that target is illuminated by RF energy in a controlled test A nonlinear radar tailored to a set of RF electronic responses would help law enforcement agents to locate devices whose emissions exceed those permitted by law, allow security personnel to detect unauthorized radio electronics in restricted areas, or enable first-responders to pinpoint personal electronics during emergencies such as immediately after an avalanche or earthquake.
In the theater-of-operations, warfighters encounter threats that contain RF electronics. Similarly, in the civilian world, there are instances when cellphones, smartphones, tablets and other “targets” are objectionable and/or dangerous Such RF electronic devices are generally small (man-portable) and may be buried or located close to the ground, making it difficult to distinguish from background clutter using traditional linear radar.
Whereas linear radar exploits the reflection from a target whose frequencies are the same as those transmitted, nonlinear radar exploits the electronic response from a target whose reflected frequencies are different from those transmitted Reception of frequencies that are not part of the transmitted probe distinguishes the received signal from a linear return that can be produced by clutter and indicates the presence of an electronic circuit. For the warfighter, the presence of an electronic circuit (in a location that typically does not contain an electronic circuit) implies the presence of a threat. Ultra-wideband (UWB) ground-penetrating radar (GPR) is a linear radar technology for detecting concealed targets such as landmines and other explosive devices. UWB GPR attempts to detect a threat set similar to that of the present invention. Since electronics and clutter both produce linear radar reflections, UWB GPR systems require a greater degree of signal processing to separate targets from clutter. By confining the detectable target response to nonlinear interactions, nonlinear radar is able to more easily separate targets from clutter.
Nonlinear radar is capable of detecting almost any un-shielded electronics, whether the electronics are on or off. Nonlinear radar exploits the electronic response from a target whose reflected frequencies are different from those transmitted. Reception of frequencies that are not part of the transmitted probe distinguishes the received signal from a linear return produced by clutter and indicates the presence of electronics. Several devices and methods exist for identifying electronics and other manmade objects using the nonlinear responses of metal and semiconductor junctions. Some detectors tune to the harmonics of a single-frequency radar transmission, such as in U.S. Pat. No. 3,732,567 to Low. Other detectors tune to the intermodulation produced by the interaction of multiple frequencies at the target, such as discussed in “A practical superheterodyne-receiver detector using stimulated emissions,” by C. Stagner, et al., in IEEE Trans. Instrum. Meas., vol. 60, no. 4, pp. 1461-1468 (April 2011) (herein incorporated by reference). In the Stagner, et al. paper, the unintended emissions of super heterodyne receivers are analyzed for the detection of radio-controlled explosives. Arbitrary signals are injected into a radio's unintended emissions using a relatively weak stimulation signal, referred to as stimulated emissions. Intermodulation products are generally the result of odd-order nonlinear interactions.
Several organizations have considered the application of radar to the problem of detecting electronic devices in secured areas. One approach envisions the use of portals to detect the unwanted devices before individuals can carry them into the restricted area See, http://tibbetts.challengepost.com/submissions/5983-vadum-transceiver-detection-for-physical-security. Other approaches consider the use of radar to detect the objects at greater distances, thereby eliminating the need for a portal. In both cases, however, the detection systems exploit a target device's non-linear, harmonic responses to a specific transmitted waveform. Since natural objects fail to produce this non-linear response, these new systems offer the opportunity to dramatically reduce the number of false alarms generated at a desired detection probability. They accomplish this through analysis of the complex magnitude of harmonics and inter-modulation (intermod) products produced by the target. In what follows attention is restricted to non-linear radar systems that are capable of detecting targets at a distance.
Researchers have recently developed radar systems capable of exploiting non-linear target responses to precisely locate targets in range. These systems typically achieve the bandwidth necessary for range resolution through transmission of either a stepped-frequency or chirped waveform. The second harmonic of the reflected waveform is then analyzed to isolate the non-linear target response. In other experiments, researchers have identified certain targets through the inter-modulation products they produce in response to a multi-tone stimulus. These experiments, however, do not exploit the phase information available in the inter-modulation products.